Field of the Invention
This invention relates generally to anti-reflective coatings and methods of depositing AR coatings, and more particularly, to a fluorinated and hydrogenated “diamond-like carbon” (DLC-FH) coating material and method of depositing same, particularly on large substrates, such as vehicle or building windows.
Description of the Prior Art
Anti-reflection (AR) coatings are applied to the surfaces of optical devices to reduce reflection, and hence, to maximize transmission of light. However, due to limitations in material properties of presently available AR coatings and in coating deposition techniques, the use of AR coatings has been restricted mainly to niche applications involving objects having comparatively small spatial dimensions, such as eye glasses, cameras, binoculars, refractive telescopes, microscopes, and the like.
There is a need, however, for AR coating materials and deposition techniques for applying AR coatings to objects having larger dimensions, such as vehicle and building windows. Vehicle and building windows are exposed to harsh environmental conditions, and therefore, it is important that any AR coatings developed for these purposes must be mechanically durable, that is, scratch and abrasion resistant, and, of course, water insoluble. There is, thus, a need for mechanically durable and water insoluble AR coating materials, and a method of applying them to large scale objects.
It is well-known to use AR coatings to enhance human comfort, such as by reducing glare in eye glasses, or to enhance the optical performance of lenses. The performance of cameras, for example, is enhanced because the AR coating permits collection of a greater amount of light under dim conditions while reducing stray light for greater image contrast. However, there is also a need for AR coating technology to enhance public safety.
A recent study published by Flannagan, et al., “Effects of Automotive Interior Lighting on Driver Vision,” LEUKOS, Vol. 9, No. 1, page 9 (July 2012), demonstrated that “veiling” light during nighttime operation of automobiles can distract the driver and reduce his ability to detect the presence of pedestrians. This veiling light originates from light sources within the automotive cabin, which reflects off the front windshield back into the driver's eyes. AR technology could find widespread application for public safety purposes, if it could be accomplished on large scale platforms, such as on vehicle windows. Reducing the effect of veiling light would enable a driver to perceive dimly lit objects outside the vehicle more quickly, thereby increasing the time for reaction. Increasing the time for reaction is key to increasing transportation safety.
In passenger automobiles, where the principal source of veiling light derives from the dashboard, this safety factor is comparatively minor because the interior light sources are relatively weak. However, veiling light distraction is particularly problematic for municipal transit systems. By regulatory mandate, the interior cabin of a bus must be illuminated to significantly greater levels. Moreover, cabin geometry is another contributing factor that exacerbates the problem in a bus versus an automobile. While the sloping windshield of an automobile helps to direct reflected interior light down and away from the driver, the nearly vertical windshield of a typical bus is ideal for directing reflected light directly toward the driver.
Altering the interior light levels in public transportation would require regulatory action and changing the slope of the windshield would require a re-design of the vehicle and public acceptance of the new aesthetics. It would be easiest to find a technological solution to mitigate against the high native reflection of uncoated glass as presently used in the vehicle windshield. Unfortunately, while the need for AR treatment of the front windshield is eminently clear, the solution to addressing this need is not.
As indicated above, there are two fundamental shortcomings of traditional AR solutions for large area applications, involving: (i) limitations to required refractive index and durability of existing materials; and (ii) limitations with the deposition methods presently employed to apply the materials.
The traditional approach to AR coatings uses quarter-wave interference layers whereby the refractive index of the AR layer [ηAR] must equal the square root of the refractive index of the glass [ηglass]. See, for example, Hecht, et al., OPTICS, (Addison-Wesley, Reading, MA, 1974), p.313. For high index substrates, like crystalline germanium used in infrared optics, where the refractive index of the substrate [ηsub] about 4.0, the [ηAR] should be about 2.0. There are many materials (including the DLC materials discussed below) which have indices of around 2 which make them suitable for use as an AR coating on a high index substrate.
It has been a challenge, however, to find materials having a sufficiently low refractive index to pair with low index substrate materials, such as the soda lime glass (SLG), commonly used for automotive windows and commercially available windows for building, which has a [ηglass] of 1.525, or translucent polymers having indices between 1.55 and 1.65. This means that [ηAR] should ideally be about 1.235. It is difficult to find materials having refractive indices lower than even about 1.34 as shown in Table 1 should ideally be about 1.235. It is difficult to find materials having refractive indices lower than even about 1.34 as shown in Table 1.
Table 1 shows a list of the five materials currently known to have low refractive index, i.e., [η]<1.4.
TABLE 1Main DepositionMaterialTechnique(s)Refractive Index (η)Calcium(CaF2)Mo or Ta boat1.23 to 1.26 at 546 nmfluorideevaporation, e-beamevaporationCryolite(Na3AlF6)Ta boat evaporation1.35 at 550 nmLithium(LiF)Ta boat evaporation1.36 to 1.27 at 546 nmfluorideMagnesium(MgF2)Ta boat evaporation1.38 at 550 nmfluorideSodium(NaF)Ta boat evaporation1.34 in visiblefluorideMolybdenum (Mo); Tantalum (Ta)MacLoed, Thin-Fim Optical Filters, 3rd Edition, (Institute of Physics, Philadelphia, 2001), p. 621
All of the low refractive index materials shown on Table 1 are fluorides, and unfortunately, would not be suitable for large area applications, such as vehicle windshields or building windows. First, there are several material property issues that fundamentally preclude their consideration. These materials tend to be soft and, therefore, would be easily scratched. Moreover, the solubility of these ionic materials in water, while low, is not zero. Therefore, they would have poor long term durability (and stability) under exposure to wet environments (e.g., fogging on the interior and exposure to snow, ice and rain on the exterior) and under the typical expected physical abuse (e.g., windshield wipers, dirt and insect impacts, hands)
Second, fluoride cannot be sputtered easily. According to Macleod, id., “Many of the [optical] materials, with the principal exception of the fluorides, can be sputtered in their dielectric form by either radio frequency sputtering or neutral ion-beam sputtering.” Unfortunately, sputter deposition is a widely used method for accurately, and cost effectively, applying thin films on substrates ranging in size from small to quite large. This alone is a major impediment to applying these known low-index materials onto very large substrates to achieve AR functionality.
There is a need, therefor, for low refractive index materials for use as AR coatings on large substrates, such as windshields and windows, which are robust enough to endure use in a harsh environment, and which can be applied economically to a large scale substrate.
In order to overcome the shortcomings of known prior art AR materials, we investigated diamond-like carbon (DLC), and in particular, chemical modifications to known DLC material involving the addition of fluorine and hydrogen. As used herein, the terms “DLC-F” or “DLC-H” refer to DLC materials that have the addition of fluorine (F) or hydrogen (H). The term “DLC-FH” has been used herein for the composition of the present invention, which is a fluorinated and hydrogenated diamond-like carbon material having advantageous physical and optical properties. It is to be understood, however, that as used in the discussion herein, the term also encompasses DLC compounds, produced in accordance with the method of the present invention, that have the equivalent advantageous physical and optical properties. In the literature, DLC materials are also referred to as amorphous hydrogenated carbon (a-C:H). See, for example, Alterovitz, et al., “Amorphous Hydrogenated ‘Diamondlike’ Carbon Films and Arc-Evaporated Carbon Films”, in Handbook of Optical Constants of Solids II, Edited by E. D. Palik (Academic, New York, 1998), p. 837.
Regardlss of the nomenclature, these materials are not to be confused with “diamond,” which is a crystalline form of carbon having purely sp3 hybridized atomic bonds between carbon atoms (C-C bonds) forming the most rigid network of three-dimensionally and tetrahedrally arranged carbon atoms. Instead, DLC or a-C:H materials are amorphous, with a mixture of sp3 and sp2 (two-dimensional trigonal arrangement of carbon atoms as found in graphene or monolayers of graphite) hybridized bonding, and which can have up to 25atom % hydrogen (in the form of C-H bonds). Nor should these materials be confused with the many different forms of soft amorphous carbon (e.g., sputtered carbon, soot, etc.) which tend to be soft by having very low, and indeed, zero sp3 bonding content. The DLC nomenclature is used to convey the fact that these materials incorporate sufficient sp3 hybridized C-C bonds to be very tough, stiff and hard, and with very low friction. In its unmodified pure carbon state, DLC materials are also commonly referred to as “tetrahedral amorphous carbon” (ta-C) to highlight the preponderance of sp3 hybridization. See, for example, Haubold, et al., “The influence of the surface texture of hydrogen-free tetrahedral amorphous carbon films on their wear performance”, Diamond Relat. Mater., Vol. 19, page 225 (2010); Yang, et al., “Electroanalytical Performance of Nitrogen-Containing Tetrahedral Amorphous Carbon Thin Film Electrodes,”Anal. Chem., Vol.84, No. 14, page 6240 (2012).
While known DLC has excellent physical attributes, it is not normally considered to be an anti-reflective material. This is because its refractive index, [ηDLC], ranges from between about 1.7 and 2.2 which is too high to match with most glass and polymer substrates. Alterovitz, supra. However, DLCs have been used in anti-reflective optical stacks to provide abrasion resistance. One known use of DLCs for this purpose is as an AR coating for mobile electronic device displays. See, Madocks, et al., “Durable Neutral Color Anti-Reflective Coating for Mobile Displays,” SVC Bulletin, p. 32, Fall 2014.
In order to demonstrate novelty of the present invention, specifically the DLC-FH composition of matter embodiment, as used for AR coatings, it is important to understand the restrictions of the use of DLC materials in an optical stack arrangement, and how multilayer optical AR stacks are designed.
Using a single thin film layer to achieve AR is the simplest case, where the refractive index of the AR layer must conform to a condition relative to the substrate, defined by equation (1), where nAR ηAR is the refractive index of the AR coating and nsub is the refractive index of the substrate,[nAR]=√{square root over (nsub×nair)},  Eqn. (1)Since the refractive index of air [ηair] is very close to 1.0, Eqn. (1) becomes the more familiar Eqn. (2)[nAR]=√{square root over (nsub)},  Eqn. (2)Since, as indicated above, the AR coating must be a quarter-wavelength thick [¼λ], the physical thickness of the AR layer [δAR] must be defined by Eqn. (3)
                                          d            AR                    =                                    1              4                        ⁢                                          λ                o                                            n                AR                                                    ,                            Eqn        .                                  ⁢                  (          3          )                    where λ0 is the design wavelength in the incident medium where the reflectance is minimized.
For high index substrates, like crystalline germanium used in infrared optics, where the [ηsub] is about 4.0, the [ηAR] should be about 2.0. There are many materials that have indices of around 2, including the DLCs. However, it has been a challenge to find materials having a sufficiently low refractive index to pair with low index substrate materials, such as the soda lime glass (SLG), commonly used in the commercial window industry, which has a refractive index of 1.525, or translucent polymers having indices between 1.55 and 1.65. This means that [nar] [ηAR]. should be about 1.235, and this represents an enormous materials challenge since it is difficult to find materials having refractive indices lower than even about 1.34 as shown in Table 1.
Another known way to produce AR coatings is through the use of two or more stacked layers of alternating low and high index materials. Basically, the refractive indices of the thin film stack configuration should be the following: [air/low /high/substrate/ . . . ]. The words “low” and “high” refer to the low index [ηlow] and high index [ηhigh] layers relative to the index of the substrate [ηsub], i.e., where [ηlow]<[ηhigh] and [ηhigh]>[ηsub]. 
H. A. Macleod, supra., at page 111, shows that the physical thickness of each layer, [dlow] and [dhigh], depends on the refractive indices, where the phase thickness (δ1) for layer-1 (the low-index layer) is given by
                                                        tan              2                        ⁢                          δ              1                                =                                                                      (                                                            n                      s                                        -                                          n                      a                                                        )                                ×                                  (                                                            n                      high                      2                                        -                                                                  n                        a                                            ⁢                                              n                        s                                                                              )                                ⁢                                  n                  low                  2                                                                              (                                                                                    n                        s                                            ⁢                                              n                        low                        2                                                              -                                                                  n                        a                                            ⁢                                              n                        high                        2                                                                              )                                ×                                  (                                                                                    n                        a                                            ⁢                                              n                        3                                                              -                                          n                      low                      2                                                        )                                                      =                          A              1                                      ,                            Eqn        .                                  ⁢                  (                      4            ⁢            a                    )                    and the phase thickness for layer-2 (the high-index layer) is given by
                                          tan            2                    ⁢                      δ            2                          =                                                            (                                                      n                    s                                    -                                      n                    a                                                  )                            ×                              (                                                                            n                      a                                        ⁢                                          n                      s                                                        -                                      n                    low                    2                                                  )                            ⁢                              n                high                2                                                                    (                                                                            n                      3                                        ⁢                                          n                      low                      2                                                        -                                                            n                      a                                        ⁢                                          n                      high                      2                                                                      )                            ×                              (                                                      n                    high                    2                                    -                                                            n                      a                                        ⁢                                          n                      s                                                                      )                                              =                      A            2                                              Eqn        .                                  ⁢                  (                      4            ⁢            b                    )                    
The phase thickness (δi) is given in terms of the physical layer thickness (d) according to Eqn. (5)
                                          δ            i                    =                      2            ⁢            π            ⁢                                                  ⁢                          n              i                        ⁢                                          d                i                                            λ                o                                                    ,                            Eqn        .                                  ⁢                  (          5          )                    
in terms of the refractive index of layer (i) and the design wavelength in the incident medium (Ia). The form of the expressions in Eqns. (4), i.e., the square of the tangent functions, shows that there are in fact two solutions for each di. In other words, Eqns. (4) can be expressed astan δ1=±√{square root over (A1)},  Eqn. (6a)andtan δ2=±√{square root over (A2)},  Eqn. (6b)Therefore, the two solutions (for each layer) areδ1±=±tan−1√{square root over (A1)},  Eqn. (7a)andδ2±=±tan−1√{square root over (A2)}.  Eqn. (7b)
FIG. 1 is a graphical representation of these positive (+) and negative (−) solutions. The negative solution seems to imply a negative thickness by Eqn. (5). However, this same negative slope (i.e., negative arctangent) can be achieved by the positive phase angle of (π−δ). Therefore, the positive solutions areδip=δio=+tan−1√{square root over (Ai)}  Eqn. (8a)and the negative solutions areδin=π−δio  Eqn. (8b)
Once it is recognized that the solutions are in terms of cyclical radians, a natural consequence is that there must be many periodic solutions, i.e., solutions which repeat every 2π radians. Evidently, the solutions in Eqn. (8) represent the zeroth order (m=0), but there must also be infinite number of solutions for m=1, 2, 3, etc. It can, therefore be shown using Eqns. (5) and (8), that all positive-solution thickness values for layer (i) are, for order (m),
                                                        d              i              p                        ⁡                          (              m              )                                =                                                    λ                o                                            n                i                                      ⁢                          1                              2                ⁢                π                                      ⁢                          (                                                2                  ⁢                  π                  ⁢                                                                          ⁢                  m                                +                                  δ                  i                  o                                            )                                      ,                            Eqn        .                                  ⁢                  (                      9            ⁢            a                    )                    and the corresponding negative-solution thickness values are
                                          d            i            n                    ⁡                      (            m            )                          =                                            λ              o                                      n              i                                ⁢                                                    1                                  2                  ⁢                  π                                            ⁡                              [                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          (                                              m                        +                                                  1                          2                                                                    )                                                        -                                      δ                    i                    o                                                  ]                                      .                                              Eqn        .                                  ⁢                  (                      9            ⁢            b                    )                    
The negative solution values, therefore, have a greater physical thickness compared to the related positive solution values. Using the appropriate spectral calculations, it has been found that the AR condition requires using the positive solution for one layer, and the negative solution for the adjacent layer. In other words, the two correct (d1, d2) sets are (dp1, dn2) and (dn1, dp2) and not (dp1, dp2) and (dn1, dn2) as previously thought.
The known multi-layer AR stack arrangement described Madocks, supra., serves as an example. Madocks used a plasma-enhanced chemical vapor deposition (PECVD) method to form SiO2 as the [ηlow] layer (˜1.45), and SiN as the [ηhigh] layer (˜1.95 to 2.1) in a stacked configuration having a total of six layers in three consecutive pairs of the high/low design. While one high/low pair can achieve AR at one specified wavelength, the more high/low pairs that are used, the greater the “band width” at which the AR is achieved. Madocks capped off the sixth layer (SiO2) with a very thin seventh layer consisting of a DLC. Since the refractive index of the DLC used by Madocks was about 2.0, the DLC layer is actually a “high ” index layer so that [ηDLC]>[ηlow]. This this means that the optical stack of Madocks ends with a high index layer, not with a low index layer as strictly required in the high/low AR design strategy outlined above.
Model calculations for the Madocks AR design are shown in FIG. 2 which is a graphical representation of the physical thickness (nm) of the high/low layers in the multi-layer AR structure comprising three sets of high/low pairs on each side of a glass substrate. As shown in FIG. 2, the thickness of the high/low layers are not the same for each of the 3 pairs that form the AR stack. Instead, once the DLC capping layer is added, it is evident that the thickness of each of these other high/low layers is modified to accommodate this disruption.
FIG. 3 is a graphical representation of the calculated visible reflectance [Rvis] of the AR structure of FIG. 2. Referring to FIG. 3, the visible reflectance [Rvis], as a function of the thickness of the DLC layer (nm), is shown for the bare glass substrate as the horizontal dotted line; for an AR stack on one side of the substrate (dashed; 1×AR); and for an AR stack on both sides of the substrate (heavy solid; 2×AR). FIG. 3 shows that the accommodation can work for very thin DLC layers, that is layers having a thickness less than about 15 nm. However, as the DLC layer thickness approaches 60 nm, the stack completely losses any AR function, with the reflectance approaching that of bare glass. Clearly, use of unmodified DLC in this concept is restricted to layers that are less than 15 nm thick as a result of the “high” refractive index.